Research On The Influence Of Mode Shapes On Wind-induced Response Of Tall Buildings

With the improvement of HFBB (high-frequency base balance) technique andassociated theory, HFBB model testing has been gradually developed since the early1970s. Compared with the aero-elastic model test as well as SMPSS (synchronousmulti-pressure scanning system) technique, HFBB technique has its obviousadvantages:(1) It is only need to consider the geometry of a tested building withlight-weight and high-strength material. Hence, the manufacture of such a model isvery simple;(2) Test method and data processing are simple and convenient.Therefore, HFBB technique has been extensively employed in the recent decades.Compared with the SMPSS technique, HFBB technique has its own shortcomings,the main assumptions of the technique are:(1) The contribution of higher modes tothe wind-induced response is negligible;(2) The fundamental mode shapes of thestructure are ideal, i.e., linear lateral modes and uniform torsion modes. However,with the development and prosperity of social economy, modern high-rise buildingsare becoming taller and more flexible. The aforementioned assumptions have been nolonger applicable for some tall buildings. In this study, combined with the SMPSStechnique, further investigation of the HFBB technique is conducted.Firstly, the effect of higher modes on the wind-induced response of tall buildingsis comprehensively examined. The fundamental modes are dominant in thedisplacement, equivalent static wind load (ESWL), base shear and base bendingmoment, while the higher modes and coupled modes can be negligible. The aboveresponses can be computed accurately only from the fundamental modes. But for theacceleration response, the fundamental modes may not be dominant and the highermodes should be considered.Secondly, the influence of nonideal mode shapes on the generalized forcespectra of tall buildings is studied systematically and integrally. Correction factors forthe generalized force spectra decrease with exponent of the mode shape. In swaymodes, correction factors are greater than unity for <1, and correction factors areless than unity for>1. For twist modes, correction factors are always less thanunity. What is more, correction equations for the generalized force spectra withexponent of average wind velocity profile and exponent of mode shape areproposed in the sway and torsional directions, respectively. Finally, the HFBB technique and SMPSS technique are compared in the overallanalysis framework for the wind-induced response. Ideal mode shapes in theconventional HFBB technique are applicable when=0.8and1.2in both thealongwind direction and the acrosswind direction. Beyond this range, the generalizedforce spectra should be corrected. It is suggested that in the torsional direction, thegeneralized force spectra should be corrected for>0.4.